I. Field of the Invention
The invention relates generally to wireless communications. More particularly, the invention relates to signal characterization in a wireless communication system.
II. Description of the Related Art
In a typical wireless communication system, a plurality of remote units communicate through a common base station. FIG. 1 is a block diagram showing a typical modern wireless communication system 10. The system is comprised of a series of base stations 14. A set of remote units 12 communicate with the base stations 14. The remote units 12 communicate with the base stations 14 over a forward link channel 18 and a reverse link channel 20. For example, FIG. 1 shows a hand-held portable telephone, a vehicle mounted mobile telephone and a fixed location wireless local loop telephone. Such systems offer voice and data services. Other modern communication systems operate over wireless satellite links rather than through terrestrial base stations.
In order for multiple remote units to communicate over a common channel, a means of multiplexing the signal onto the forward link and reverse link channels must be used. One commonly used method is code division multiple access (CDMA). Additional information concerning CDMA is set forth in the TIA/EIA Interim Standard entitled "Mobile Station--Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System," TIL/EIA/IS-95-A, and its progeny, the contents of which are incorporated herein by reference. In a CDMA system, the forward and reverse link signals are modulated with a spreading code which spreads the signal energy over a band of frequencies. By correlating the incoming signal with the spreading sequences used in the transmitting units, the signals which are transmitted in the same frequency band at the same time can be distinguished from one another at the receiving unit.
In general, CDMA systems operate most efficiently when each remote unit receives the forward link signal at the minimum signal quality which is necessary in order to accurately decode the incoming signal. If the forward link signal arrives at the remote unit at a level that is too low, the signal level may not be sufficient to support reliable communications. If the forward link signal arrives at the remote unit at a level that is too high, the signal acts as unnecessary interference to other remote units. Therefore, the remote unit monitors the signal quality at which the signal is received and requests an increase in the power level at which the base station transmits the forward link signal if the signal quality is too low and requests a decrease in the power level at which the base station transmits the forward link signal if the signal quality is above the threshold.
In order to implement such a system, in one embodiment, the remote unit estimates the forward link signal quality by determining the signal-to-noise ratio at which it receives the forward link signal. The signal-to-noise ratio can be determined by finding the ratio of the energy per bit to the non-orthogonal noise power density E.sub.b /N.sub.t). The energy per bit is a measure of the energy associated with a single information bit. Typically, signal-to-noise ratios are determined over a series of bits so that an average energy per bit is determined and used as the numerator of the signal-to-noise ratio.
FIG. 2 is a block diagram of a receiver which determines an average energy per bit. A decoder 30 receives a signal vector r corresponding to a series of N symbols which make up a frame such that r=(r.sub.1, r.sub.2, . . . , r.sub.N). Each symbol, r.sub.n, is comprised of a signal portion and a noise portion as shown in Equation 1 below. EQU r.sub.n =s.sub.n +w.sub.n (Eq. 1)
wherein:
r.sub.n is a voltage value of the n.sup.th symbol; PA1 s.sub.n is the signal portion of the n.sup.th symbol in volts; and PA1 w.sub.n, is the noise portion of the n.sup.th symbol in volts. The signal component of each bit sample can be expressed in terms of a voltage level and a polarity as shown in Equation 2. EQU r.sub.n =A.sub.n d.sub.n +w.sub.n (Eq. 2) PA1 A.sub.n is the absolute value of the voltage level of the n.sup.th symbol; and PA1 d.sub.n represents the polarity (i.e., digital value) of the n.sup.th symbol (i.e., +/-1). In a digital representation, the voltage level A.sub.n is transmitted into a numerical value represented by digital bits.
wherein:
Referring again to FIG. 2, the decoder 30 receives the symbols corresponding to a frame represented by the vector r and converts them to a series of bits. In one embodiment, the decoder 30 is a Viterbi decoder. Typically, the bits output by the decoder 30 are passed to subsequent processing stages (not shown) in order to recreate a transmitted signal. In order to determine the energy associated with the signal energy in the frame, the bits output by the decoder are re-encoded by a re-encoder 32 which operates in a complimentary manner with the decoder 30 such that the output of the re-encoder 32 is the vector d=(d.sub.1, d.sub.2, . . . d.sub.N) where d.sub.n represents the polarity of the n.sup.th symbol as defined above.
The vector r and the vector d are input into a dot product block 34. The dot product block 34 takes the dot product of the two inputs as shown in Equation 3 below. ##EQU1##
The output of the square of the dot product block 34 is coupled to a squaring block 36 yielding the result given in Equation 4. ##EQU2##
Note that d.sub.n.sup.2 =1 for all n. We can also assume that the noise component of the vector r is a series of independent and identically distributed random variables with zero mean, possibly Gaussian distribution, and, thus, according to well-known principles of stochastic processes, randomly multiplying the individual components by +/-1 does not change the characteristics or average value of the noise. In this way, Equation 4 reduces to Equation 5A as shown below. ##EQU3##
The second term of Equation 5A is, by definition, the mean noise component of the vector r and is equal to zero such that Equation 5A reduces to Equation 5B as shown below. ##EQU4##
Thus, the output of the square of the dot product block 34 shows the sum of the energy of the symbols in the frame which is directly related to the energy in each bit of the frame as shown in Equation 6 below. EQU E.sub.b =(r.multidot.d).sup.2 /B (Eq. 6)
wherein: B is the number of bits in a frame.
In order to determine the signal-to-noise ratio, an estimate of the noise component of the signal must also be determined. In general, we are only interested in the non-orthogonal portion of the noise, N.sub.t, because any orthogonal portion of the noise can be removed by signal processing. Non-orthogonal noise sources include thermal noise, forward link transmissions from neighboring base stations and multipath propagations from the servicing base station. Estimation of the non-orthogonal component of the noise is more difficult than the estimation of the bit energy in general. Although several techniques have been discussed, they tend to be inaccurate or require an excessive amount of processing resources. For example, one means of determining the non-orthogonal noise energy is disclosed in U.S. Pat. No. 5,754,533 entitled "METHOD AND SYSTEM FOR NON-ORTHOGONAL NOISE ENERGY BASED GAIN CONTROL." According to one embodiment of the patent, a pilot channel or other known channel is demodulated and used to determine the non-orthogonal noise level. In such a case, a separate demodulation process is carried out for each multipath component of the signal. Based on the result of the demodulations, a noise component is separately measured for each multipath. The use of a pilot signal increases the costs of the system and decreases the capacity of the system. The demodulation of each separate multipath occurrence and the individual calculations consume significant system resources.
Therefore, there has been a long-felt need in the industry for an efficient determination of non-orthogonal noise characteristics in a digital communication system.